Europe, but I don't know how easy it would be. The game's premise is so daunting as is.
I'd rather they focus area-wise and flesh out a bit in a new region (perhaps not all of Europe, but say, Italy), but extending to Canada (my home!) would be great, as it'd offer a ton of new types of terrain.
Really, they could get by on re-doing the whole of the US with greater fidelity and I'd be in. Would be interesting if they had a model where they could continually keep updating the landscapes and cities, or plopping down new cities with updates.
This and a more straighforward, less RPG-esque upgrade system and the franchise has a ton of potential.
That said, I'm pretty excited. The game has a lot of flaws, yes (and seemingly a lot of gamers are pretty intolerant of B-games nowadays), but its pretty much the GOAT game for exploration and discovering little and big things as you drive. There is a sense of wonder that only The Crew has captured.